The present invention relates, in general, to flash memory devices and, more particularly, to a flash memory device and a method of controlling a program voltage, in which an Increment Step Pulse Program (ISPP) voltage provided for program and verification can be controlled according to a cell characteristic.
In general, a flash memory device performs a program operation, an erase operation and a read operation. The program operation of the flash memory device is executed on a page basis. In this operation, memory cells are programmed by applying a word line bias voltage to word lines to which the memory cells of a selected page are connected.
The ISPP programming method programs a selected page several times while gradually increasing a word line bias voltage on a step-voltage basis.
In the ISPP method, in order to program memory cells corresponding to one page, programming is performed by applying a first voltage Vp1 to a word line at a first time T1. For program verification, a verify voltage Vf1 is then applied to the word line at a second time T2. Thereafter, at a third time T3, a second voltage Vp2 is applied to the word line in order to perform the programming. The verify voltage Vf1 is then applied at the second time T2 for program verification. As described above, the bias voltages Vp applied to the word line on a program-step basis are sequentially applied at a predetermined amount.
If the programming is performed by applying voltage to the word line using the ISPP method, a wide distribution of cell voltages results due to the same bias step. Accordingly, it is difficult to apply the ISPP method to MLCs requiring a narrow cell voltage distribution.